There has been known commingled yarns containing continuous reinforcing fibers and continuous thermoplastic fibers (also referred to as composite fibers) (Patent Literature 1, Patent Literature 2, and Patent Literature 3).
For example, Patent Literature 1 has described a method of obtaining a composite fiber by treating a reinforcement multi-filament having substantially no oil agent or sizing agent adhered thereon, and a thermoplastic multi-filament used as a base, under predetermined conditions when both filaments are to be wound together (Claim 1, etc. of Patent Literature 1). Patent Literature 1 also discloses a method of plasticizing the thermoplastic filament in the composite fiber by heating, to thereby semi-fuse or fuse it with the reinforcement multi-filament.